Wednesday, March 12, 2008

Pretend You’re an Airplane

The next time you find yourself at the airport -- standing in the security line with nothing to do -- I have a mental exercise for you. Pretend you’re an airplane and the metal detector portal is a runway. It’s a simple exercise but you’ll be surprised how much you can learn from it.

Let’s start off with only one metal detector portal. As you intuitively understand, the simple solution is to have one line. One portal -- one line. But someone has a better idea. Instead of a line that snakes down the hallway and blocks the entrance to the concession stand, it is decided we will have two lines. This looks better because the line isn’t as long. It isn’t any better though because is takes more time to blend the two lines back together as two people stand in front of the metal detector saying, “Please, you first” and “No, you go ahead” and “No, I insist, you go first.” (I fly out of Atlanta. I realize the scene in New York might be different.)

In simple terms, this is what President Bush came up with when he announced the “express lanes” during the holidays. The two lines looked better than a single line but without more runways, airplanes weren’t landing any faster and it just made the controller’s job more difficult -- blending the two streams of airplanes into one.

This concept is very easy for you to understand -- watching it as you are -- standing in the security line at the airport. I would be the last one to tell you that air traffic control isn’t complicated but everything about it isn’t that complicated. It’s easy to understand that it takes a second or two to walk the few steps through the metal detector portal. It is just as easy to understand it takes a minute for an airplane to touch down, slow down and exit the runway. It is also easy to understand that, with a certain number of people standing in line, two metal detector portals would allow you to handle them twice as fast as just one.

Again, it’s just plain old-fashioned common sense that, if you were to put a second metal detector in place, you would place it where it wouldn’t interfere with the first one. Assuming they were to handle the same gates, you would place them parallel to each other, far enough apart that they wouldn’t interfere with each other. Sounds pretty simple right ? Well, airports aren’t that simple.

Airplanes need to land into the wind. That is the reason you’ll find airports with runways that are at angles to each other. Let me show you one of the simpler ones -- LaGuardia (LGA.)

As you can see, LaGuardia has two runways that are perpendicular to each other. Even worse (as far as air traffic control) they intersect each other. Going back to our security line/metal detector portal analogy, it isn’t real hard to figure out that this arrangement is less than ideal. Just as you are about to approach the metal detector portal/runway, you have to cross a line of people/airplanes headed for the other metal detector portal/runway. If you bump into someone as you cross the line you can just say “Excuse me” and move on. Obviously, the consequences aren’t quite the same if you’re an airplane.

We’re at a point now where we can follow thoughts in different directions if you so choose. I mentioned that airplanes need to land into the wind. There is a certain amount of flexibility to that general rule. For instance, if the wind is calm enough, the airplanes at LGA can use both runways at the same time. It allows us to run more airplanes per hour but as I’ve shown above, not as many as if the runways were parallel to each other. And it doesn’t take a genius to figure out that “shooting the gaps” between the two lines isn’t as efficient (or as safe) as using two lines that are parallel to each other. To be fair, when those runway layouts were designed, capacity wasn’t quite the Siren it is today.

We could follow the thought of how long the line would get if two runways worth of airplanes could only use one runway (because the wind was blowing so hard) but the thought I would like to follow is that all runway layouts are not equal. Yet, some people would have you believe things are simpler than they really are. Read this statement from a Port Authority of New York & New Jersey press release back in October of last year.

” The FAA has proposed a cut in the maximum number of flights at the airport to 80 an hour – equivalent to the cap at JFK in the late 1960s. Under the restriction, JFK would handle fewer flight operations per day than LaGuardia Airport, despite JFK having approximately 44,000 total feet of runway space compared to LaGuardia’s 14,000.“

To the uninformed, that statement would make it seem as if JFK should be able to handle a lot more traffic than LGA. Under the best of conditions, LGA can handle 44 arrivals per hour and JFK can handle 68. Conditions are not always the best though. It rains, it snows and sometimes, the wind just blows too hard or from the wrong direction. If you’ll look at the charts at the links provided, you’ll see that when the weather gets bad -- the IFR (Instrument Flight Rules) column on the charts -- the differences starts shrinking. Much of it can be explained by looking at the airport layout. (You can read about other factors here.)

JFK has two main runways that are parallel. That is good. Actually, it’s great. Then it has another set of parallels for when the wind blows in a different direction. That is good too. The statement from the Port Authority above would have you infer that all four of those runways can be used at the same time.

Let’s go back to our security line at the metal detector machine and check this out. Two lines to two metal detector portals are stretched out parallel -- we’ll say south to north. Then we have two other parallel lines headed for two other metal detector portals and these lines run east to west. To make this work you don’t have to “shoot the gap” once but twice. Any questions ?

For those of you who do have a question, watch this video.

Remember, I didn’t say it wasn’t complicated. This exercise is only in two dimensions but controllers work in three dimensions. The pressure to maximize the use of all runways might be overpowering for the Port Authority (and some other entities I could name) but controllers get to see the results firsthand when things go wrong. And sooner or later, something always goes wrong. Controllers want enough wiggle room to safely handle the traffic even when things don’t go exactly as planned.

I feel compelled to restate that I never worked in an Air Traffic Control Tower. I know that people forget that fact. Not having worked in a Tower, I’m sure there are some subtleties that I miss about airport operations. If you want to understand more than the basics, you need to ask the experts -- the folks that work in the Towers.

What kind of air traffic controller was I ? I was a Center controller. The next time you are standing in the security line at the airport, turn around. You see all those people coming in the doors, up the escalators and down the elevators to get in line behind you ? Pretend they’re all airplanes...

Don Brown
March 12, 2008

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